1. Field of the Invention
This invention relates to composite golf club shafts and more specifically to golf club shafts constructed from high strength unidirectional fibers incorporated in a matrix of thermoplastic resin. The unidirectional high strength fibers may include fiberglass, carbon or other well known high strength reinforcing fibers impregnated with thermoplastic resin including nylon, polyphenylene sulfide (PPS) and other well known thermoplastic resins. This invention also relates to the method of forming golf club shafts with thermoplastic unidirectional prepregs and the apparatus used for making such golf club shafts.
2. Description of the Prior Art
Golf club shafts have conventionally been constructed as a tapered hollow metal member with the lower end connected to an upwardly projecting hosel extending upwardly from the golf club head. With the exception of putters, the shaft extends from an upper edge of the club head at a point near the heel of the club head. The upper end of the club shaft has a hand grip to enable a golfer to effectively grip, manipulate and control movement of the golf club head for optimum impact with a golf ball.
Golf club shafts of composite material utilizing high strength fibers in a matrix are well known. Usually, reinforced composite material prepregs made with uncured thermoset epoxy are wrapped onto an elongated mandrel having the desired shape of the interior of the finished shaft. Once wrapped, the thermosetting epoxy is cured to form the final shape of the shaft. U.S. Pat. No. 5,093,162 discloses various arrangements of the high strength fibers in a composite golf club shaft utilizing an epoxy matrix and arrangements of unidirectional high strength fibers to provide necessary physical characteristics of the golf club shaft. The laminations or plies of the fiber reinforced epoxy matrix prepreg are wrapped around the mandrel, after which a pressure tape is wrapped around the rolled up plies. The thermosetting epoxy resin is then cured, and the hollow shaft is removed from the mandrel. The shaft may be sanded or otherwise finished.
The current practice in forming a golf shaft from thermoset carbon fiber unidirectional prepregs includes the cutting of the unidirectional prepreg material into a plurality of individual pieces of specific configuration. The individual pieces are then rolled onto a metal mandrel, such as steel or the like, having the shape of the finished inside dimensions of the golf club shaft. Since thermoset prepreg carbon/epoxy material is sticky at or slightly above room temperature, about 70.degree.-100.degree. F. (about 20.degree.-40.degree. C.), the first layer will readily adhere to the mandrel, and the next layer to the preceding layer, etc. A rolling table is typically used to roll the thermoset prepreg pieces onto the mandrel.
Mandrels used to make golf club shafts usually have a straight linear taper in the central portion with cylindrical end portions of different diameters. The butt end has a larger diameter than the tip end, and the central portion typically has a gradual straight taper therebetween.
In making thermoset golf shafts, four or more separate pieces of prepreg usually extend the full length of the shaft. Normally, the unidirectional fibers are oriented in an opposed biased relationship. For example, as disclosed in the aforesaid U.S. Pat. No. 5,093,162, ply one may have fibers at a +45.degree. orientation, ply two -45.degree., ply three 0.degree. and ply four 0.degree., with 0.degree. being the longitudinal axis of the mandrel.
The following U.S. patents disclose various developments in the formation of tubes or hollow shafts including golf club shafts using composite material with unidirectional high strength fibers and a thermosetting resin matrix in the form or a prepreg wrapped on a mandrel and cured with the formed tube or shaft then being removed from the mandrel for final finishing:
U.S. Pat. Nos. 4,097,626, 5,308,062, 5,421,573, 5,262,118, 5,326,099, 5,439,219
There have also been attempts to make carbon fiber golf club shafts with thermoplastic resins, see for example U.S. Pat. No. 5,262,118. In these other examples the thermoplastic resin is in discontinuous form which permits the carbon fiber sheet to remain sufficiently flexible so that it can be rolled onto a mandrel below its melt temperature. The disadvantage to these particular processes is that the finished shaft has many voids in it due to the discontinuities in the prepreg. These voids are deleterious, from both a strength and a performance point of view.